A linear regulator, also referred to as a low dropout (LDO) regulator, controls the conductance of a bipolar or MOS transistor in series between a supply voltage and the output terminal of the regulator. The conductance of the transistor is controlled based upon the difference between a feedback voltage (corresponding to the output voltage) and a reference voltage to keep the output voltage at the desired level. An LDO regulator provides a very smooth output by constantly adjusting the conductivity of the series transistor.
In contrast to an LDO regulator, a switching regulator, such as a pulse width modulation (PWM) regulator, switches a transistor on and off at a variable duty cycle so the output voltage, even though filtered, has some undesired ripple. Switching regulators are generally more efficient than LDO regulators at medium to high currents since the switching transistor operates in or near saturation and energy is transferred efficiently from the supply voltage to the output through a reactive element, such as an inductor.
LDO regulators are most efficient when the supply voltage applied to the series transistor is only slightly greater than the output voltage so as to minimize the voltage drop across the series transistor. To maximize efficiency, the series transistor should operate at the edge of saturation, such as just outside of saturation, to minimize power dissipation in the transistor while simultaneously compensating for variations in the supply voltage.
One approach that has been used in the past is to provide an efficient switching regulator, such as a pulse width modulation (PWM) regulator, that generates a supply voltage for the LDO's series transistor that is only slightly above the desired output voltage of the LDO. However, since the dropout voltage of the series transistor varies with current, process variations, temperature, and other factors, the supply voltage must be set to be at or above that voltage needed for the worst case scenario. Therefore, the supply voltage is typically higher than what is needed under normal conditions, reducing the efficiency of the LDO regulator.
What is needed is a more efficient voltage regulator using a switching regulator to supply voltage to the LDO series transistor.